Plasma picture screen with UV light reflecting front plate coating

ABSTRACT

A plasma picture screen is disclosed. In one embodiment, an AC plasma picture screen with a coplanar arrangement which has an enhanced luminance is provided. A layer, which has a high reflection in the wavelength range of the plasma emission (145 to 200 nm) and a high transmission in the visible wavelength range, is provided on the front plate. The front plate includes a glass plate on which a dielectric layer, a protective layer, a reflective layer are provided. The reflective layer reflects UV light emitted in the direction of the front plate back towards phosphors. The optical properties of the UV light reflecting layer are realized with inorganic particles with a particle diameter of between 200 nm and 500 nm and a layer thickness from 0.5 μm to 5 μm, or with agglomerates of inorganic particles with a particle diameter of between 10 nm and 200 nm and a layer thickness of 0.2 μm to 10 μm.

The invention relates to a plasma picture screen with a front platecomprising a glass plate on which a dielectric layer and a protectivelayer are provided, a back plate, a number of gas-filled plasma cellsarranged between said plates and separated by partitioning walls, and aplurality of electrodes on the front plate and the back plate forgenerating corona discharges.

Plasma picture screens can generate color pictures with high resolutionand have a compact construction. A plasma picture screen comprises ahermetically closed glass cell which is filled with a gas, withelectrodes in a grid arrangement. When a voltage is applied, a gasdischarge is generated which emits light in the ultraviolet range. ThisUV light is converted into visible light by phosphors and emittedthrough the front plate of the glass cell to the viewer.

Two types of plasma picture screens can be distinguished in principle: amatrix arrangement and a coplanar arrangement. In the matrixarrangement, the gas discharge is ignited and maintained at theintersection of two electrodes on the front and back plate. In thecoplanar arrangement, the gas discharge is maintained between theelectrodes on the front plate and is ignited at the intersection with anelectrode, a so-called address electrode, on the back plate. The addresselectrode in this case lies below the phosphor layer. This arrangementimplies that half the UV light generated in the gas discharge willarrive at the front plate, where it is absorbed in the layers presentthere. This effect is even enhanced for a portion of the UV light,because the UV light is re-absorbed in the gas space in that gas atomsare excited from the ground state into a higher energy level. The lightis indeed emitted again subsequently, but it is diverted from itsoriginal direction, so that also light which had originally beendirected towards the phosphor layers can arrive on the front plate.

The luminance of a plasma picture screen depends to a major extent onthe efficacy with which the UV light excites the phosphors. To increasethe luminance, T. Murata, Y. Okita, S. Kobayashi, T. Shinkai, and K.Terai describe a plasma picture screen in IEEJ, 1998, EP 98-61, whereinthe front plate comprises not only a dielectric layer and the dischargeelectrodes, but also a UV light reflecting layer. A similar constructionof a front plate was described by the same authors in IDW, 1998,539-542. In this case the front plate in addition comprises a protectivelayer of MgO, and the UV light reflecting layer is present between thedielectric layer and the protective layer. In either case said layer hasthe task of reflecting UV light emitted in the direction of the frontplate towards the phosphors.

The invention has for its object to provide a plasma picture screen withan improved luminance.

This object is achieved by means of a plasma picture screen with a frontplate comprising a glass plate on which a dielectric layer and aprotective layer are provided, a back plate, a number of gas-filledplasma cells arranged between said plates and separated by partitioningwalls, and a plurality of electrodes on the front plate and the backplate for generating corona discharges, characterized in that a UV lightreflecting layer is provided on the protective layer.

Contrary to the front plate described in IDW, 1998, 539-542, the UVlight reflecting layer does not lie between the dielectric layer and theprotective layer. This has the advantage that the UV light does not passthrough the protective layer, but is reflected directly at the lowersurface of the front plate. An absorption of the UV light in theprotective layer is prevented thereby. The additional protective layerin the plasma picture screen according to the invention as compared withthat of IEEJ, 1998, EP 98-61 protects the subjacent layers from the highignition voltages and temperatures which are required for a plasmadischarge and arise in the plasma discharge, respectively.

It is particularly preferred that the UV light reflecting layercomprises oxides of the composition M₂O, such as Li₂O, or oxides of thecomposition MO, with M chosen from the group Mg, Ca, Sr, and Pa, oroxides of the composition M₂O₃, with M chosen from the group B, Al, Sc,Y, and La, or oxides of the composition MO₂, with M chosen from thegroup Si, Ge, Sn, Ti, Zr, and Hf, or oxides of the composition M′M″₂O₄,with M′ chosen from the group Mg, Ca, Sr, and Ba, and M″ chosen from thegroup Al, Sc, Y, and La, or fluorides of the composition MF, with Mchosen from the group Li, Na, K, Rb, Cs, and Ag, or fluorides of thecomposition MF₂, with M chosen from the group Mg, Ca, Sr, Ba, Sn, Cu,Zn, and Pb, or fluorides of the composition MF₃, with M chosen from thegroup La, Pr, Sm, Eu, Gd, Yb, and Lu, or fluorides of the compositionM′M″F₃, with M′ chosen from the group Li, Na, K, Rb, and Cs, and M″chosen from the group Mg, Ca, Sr, and Ba, or phosphates of thecomposition M₃PO₄, with M chosen from the group Li, Na, K, Rb, and Cs,or phosphates of the composition M₃(PO₄)₂, with M chosen from the groupMg, Ca, Sr, and Ba, or phosphates of the composition MPO₄, with M chosenfrom the group Al, Sc, Y, La, Pr, Sm, Eu, Gd, Yb, and Lu, or phosphatesof the composition M₃(PO₄)₄, with M chosen from the group Ti, Zr, andHf, or metaphosphates with a chain length n of between 3 and 9 and thecomposition (M_(x)PO₃)_(n), with x=1 if M is chosen from the group Li,Na, K, Rb, and Cs, x=½ if M is chosen from the group Mg, Ca, Sr, Ba, Sn,Cu, Zn, and Pb, x=⅓ if M is chosen from the group Al, Sc, Y, La, Pr, Sm,Eu, Gd, Yb, and Lu, and x=¼ if M is chosen from the group Ti, Hf, andZr, or polyphosphates with a chain length n between 10¹ and 10⁶ and thecomposition (M_(x)PO₃)_(n), with x=1 if M is chosen from the group Li,Na, K, Rb, and Cs, x=½ if M is chosen from the group Mg, Ca, Sr, Ba, Sn,Cu, Zn, and Pb, x=⅓ if M is chosen from the group Al, Sc, Y, La, Pr, Sm,Eu, Gd, Yb, and Lu, and x=¼ if M is chosen from the group Ti, Hf, andZr, or primary phosphates of the composition MH₂PO₄, with M chosen fromthe group Li, Na, K, Rb, and Cs, or NH₄H₂PO₄, or diamond.

Particles with these compositions show no or only a small absorption inthe wavelength range from 147 to 700 nm and withstand the hightemperatures prevailing during manufacture of a plasma picture screen.In addition, these particles can be manufactured with particle diametersbetween 10 nm and 500 nm.

It may be preferred that the UV light reflecting layer comprisesparticles with a particle diameter between 200 nm and 500 nm.

Particles of these diameters show a substantially greater lightscattering in the UV wavelength range than in the visible wavelengthrange.

It is preferred in this embodiment that the UV light reflecting layerhas a thickness of 0.5 μm to 5 μm.

Besides the scattering characteristics of the individual (isolated)particles and their wavelength dependence, the thickness of the layer ofscattering particles also plays a part. Thus the reflection of a layerof particles which are less strongly scattering can be high if the layerthickness is great. The use of particles with a particle diameterbetween 200 nm and 500 nm and a layer thickness of 0.5 μm to 5 μmresults in a UV light reflecting layer which reflects strongly in thewavelength range of the plasma emission and transmits the visible lightemitted by the phosphors.

It may also be preferred that the UV light reflecting layer comprisesagglomerates of particles having particle diameters between 10 nm and200 nm.

Particles with particle diameters between 10 nm and 200 nm do notscatter UV light, so that layers made from such particles show noappreciable reflection. If it is ensured by means of suitable measuresthat the particles form agglomerates which are substantially greaterthan 100 nm, the layer will behave optically as if it were a layer oflarger particles. The interaction between the light and the agglomeratesis determined both by the size of the agglomerates and by the particlediameters of the particles forming the agglomerates. The variations indensity and refractive index in the agglomerates also has an influenceon the wavelength dependence of the reflection.

It is preferred in this embodiment that the UV light reflecting layerhas a thickness of 0.2 μm to 10 μm.

It is furthermore preferred that the UV light reflecting layer coversthe protective layer completely or only partly.

A partial covering of the protective layer with the UV light reflectinglayer already leads to a considerable improvement in the luminance. Itmay be advantageous for the plasma discharge if the protective layer,which is usually made of MgO, is covered only partly. This is becausethe voltage required for igniting the plasma is reduced by the MgOlayer.

The invention will be explained in more detail below with reference totwo Figures and eight embodiments, in which

FIG. 1 shows the construction and operating principle of a single plasmacell in an AC plasma picture screen, and

FIG. 2 plots the reflection characteristics of a UV light reflectinglayer according to the invention.

In FIG. 1, a plasma cell of an AC plasma picture screen with a coplanararrangement comprises a front plate 1 and a back plate 2. The frontplate 1 comprises a glass plate 3 on which a dielectric layer 4 andthereon a protective layer 5 are provided. The protective layer 5 ispreferably made of MgO, and the dielectric layer 4 is made, for example,of glass comprising PbO. Parallel, strip-shaped discharge electrodes 6,7 are provided on the glass plate 3 such that they are covered by thedielectric layer 4. The discharge electrodes 6, 7 are made, for example,from metal or ITO. A UV light reflecting layer 8, which reflectsradiation 12 in the UV range and transmits visible light 14, is providedon the protective layer 5. The back plate 2 is made of glass, andparallel, strip-shaped address electrodes 11, for example made of Ag,are provided on the back plate 2 so as to run perpendicularly to thedischarge electrodes 6, 7. Said address electrodes are covered withphosphor layers 10 in one of the three basic colors red, green, or blue.The individual phosphor layers 10 are separated by partitioning walls13, so-called barriers, which are preferably made of a dielectricmaterial.

A gas, preferably a mixture of rare gases, for example He, Ne, Xe, orKr, is present in the discharge cavity and also between the dischargeelectrodes 6, 7, of which one is the cathode and the other the anode,alternately. After the surface discharge has been ignited, such thatcharges can flow along a discharge path which lies in the plasma range 9between the discharge electrodes 6, 7, a plasma is formed in the plasmarange 9 whereby radiation 12 is generated in the UV range, in particularin the VUV range, in dependence on the composition of the gas. This UVradiation 12 excites the phosphor layers 10 into phosphorescence, sothat visible light 14 in the three basic colors is emitted and issuesthrough the front plate 1 to the exterior, thus forming a luminous pixelon the picture screen.

The dielectric layer 4 lying over the transparent discharge electrodes6, 7 in AC plasma picture screens serves inter alia to prevent a directdischarge between the discharge electrodes 6, 7 of conductive material,and thus the formation of an arc during ignition of the discharge.

To manufacture a front plate 1 with a UV light reflecting layer 8, thedischarge electrodes 6, 7 are first provided by means of a vapordeposition process on a glass plate 3 whose size corresponds to thedesired picture screen size. Then a dielectric layer 4 is provided, anda protective layer 5 on the dielectric layer 4.

Suspensions of the particles with the desired particle diameter arefirst prepared for the UV light reflecting layer 8. Particles which maybe used are, for example, oxides, fluorides, phosphates, metaphosphates,or polyphosphates of various main group or subgroup metals. Oxides usedmay be, for example, the oxides of the first main group such Li₂O oroxides of the second main group such as MgO, CaO, SrO, and BaO, oroxides of the third main group such as, for example, B₂O₃ and Al₂O₃, oroxides of the third sub-group such as Sc₂O₃, Y₂O₃, and La₂O₃, or oxidesof the fourth main group such as, for example, SiO₂, GeO₂, and SnO₂, oroxides of the fourth sub-group such as TiO₂, ZrO₂, and HfO₂, or mixedoxides such as MgAl₂)₄, CaAl₂O₄, or BaAl₂O₄. Fluorides used may be, forexample, fluorides of the first main group such as LiF, NaF, KF, RbF,and CsF, or fluorides of the first sub-group such as AgF, or fluoridesof the second main group such as MgF₂, CaF₂, SrF₂, and BaF₂, orfluorides of the fourth main group such as SnF₂ and PbF₂, or fluoridesof the first sub-group such as CuF₂, or fluorides of the secondsub-group such as ZnF₂, or fluorides of the lanthanides such as LaF₃,PrF₃, SmF₃, EuF₃, GdF₃, YbF₃, and LuF₃, or mixed fluorides such asLiMgF₃ and KMgF₃. Phosphates used may be, for example, phosphates of thefirst main group such as Li₃PO₄, Na₃PO₄, K₃PO₄, Rb₃PO₄, and Cs₃PO₄, orphosphates of the second main group such as Mg₃(PO₄)₂, Ca₃(PO₄)₂,Sr₃(PO₄)₂, or Ba₃(PO₄)₂, or phosphates of the third main group such asAlPO₄, or phosphates of the third sub-group such as ScPO₄, YPO₄, andLaPO₄, or phosphates of the lanthanides such as LaPO₄, PrPO₄, SmPO₄,EuPO₄, GdPO₄, YbPO₄, and LuPO₄, or phosphates of the fourth sub-groupsuch as Ti₃(PO₄)₄, Zr₃(PO₄)₄, and Hf₃(PO₄)₄. Metaphosphates with a chainlength n of between 3 and 9 may be, for example, metaphosphates of thefirst main group such as Li₃(PO₃)₃, Na₃(PO₃)₃, K₃(PO₃)₃, Rb₃(PO₃)₃, andCs₃(PO₃), or metaphosphates of the second main group such as Mg(PO₃)₂,Ca(PO₃)₂, Sr(PO₃)₂, and Ba(PO₃)₂, or metaphosphates of the third maingroup such as Al(PO₃)₃, or metaphosphates of the third sub-group such asSC(PO₃)₃, Y(PO₃)₃, and La(PO₃)₃, or metaphosphates of the fourthsub-group such as Ti₃(PO₃)₄, Zr₃(PO₃)₄, and Hf₃(PO₃)₄, or Zn(PO₃)₂.Polyphosphates used may be, for example, polyphosphates (M_(x)PO₃)_(n)of the metals Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Al, Sc, Y, La, Ti, Zr,Hf, Zn, Pr, Sm, Eu, Gd, Yb, or Lu, with a chain length n of between 10¹and 10⁶ and with a value for x which lies between 0.25 and 1 independence on the oxidation value of the metal used. Metal cations maybe partly replaced with protons in all these polyphosphates.Alternatively, however, primary phosphates such as, for example, KH₂PO₄,NaH₂PO₄, and NH₄H₂PO₄, or diamond may be used in the UV light reflectinglayer 8.

Alternatively, the suspensions may also comprise precursors of theparticles according to the invention, which are then converted into thedesired particles by a thermal treatment. Thus, for example, asuspension with Mg(OH)₂ can be thermally converted into an MgO layerafter being provided on the protective layer 5.

The suspensions are preferably prepared in an aqueous solution. It maybe necessary in a number of cases to work with organic solvent systems,for example if the powder used reacts chemically with water or isdissolved therein.

The preparation of the suspensions takes place by various methods independence on material an d particle size. A possibility is that theparticles are synthesized from suitable precursors. The alternativepossibility is that the particles are directly utilized.

Metal salts are first dissolved in water for the situation in whichparticles are prepared from precursors in the preparation of thesuspensions. The metal salts have the composition MX_(n)·yH₂O, in whichM is, for example, a metal or several metals chosen from the group Li,Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Al, Sc, La, Y, Sn, Ti, Zr, Hf, Ag, Pb,Cu, Pr, Sm, Eu, Gd, Yb, Lu, and Zn. X is, for example, one or several ofthe anions NO₃ ⁻; RO⁻, ⁻O₂C—CO₂ ⁻; while y is a number greater than orequal to zero, and n is an integer number between 1 and 4, depending onthe oxidation value of the metal cation M^(n+). Alkoxides RO⁻ used maybe, for example, propoxide and butoxide.

The particles according to the invention with a particle diameter ofbetween 200 nm and 500 nm are then obtained either through a thermaltreatment such as, for example, heating in a reflow process, by an acidtreatment such as, for example, the addition of acetic acid, by analkaline treatment such as the addition of soda lye, or the introductionof ammonia gas and/or the addition of the desired complementary ion. Thecomplementary ions are added to the aqueous metal salt solution in theform of salts and may be, for example, ammonium salts such as NH₄F, orphosphates such as sodium metaphosphates, or long-chain polyphosphatesalts.

The suspensions thus obtained are mixed with an associative thickenerand/or a dispersing agent.

Alternatively, particles such as, for example, Li₂O, MgO, CaO, SiO, BaO,B₂O₃, Al₂O₃, Sc₂O₃, Y₂O₃, La₂O₃, SiO₂, GeO₂, SnO₂, TiO₂, ZrO₂, HfO₂, orMgAl₂O₄ with a particle diameter of between 200 nm and 500 nm may bedirectly suspended in the aqueous solution and subsequently mixed withan associative thickener and/or a dispersing agent.

If the particles used have an average particle diameter smaller than 200nm, they will be agglomerated in a controlled manner. For this purpose,the powders of the relevant particles are dispersed in a liquid phase,which is preferably an aqueous solution. The liquid phase also containsadditives which influence the colloidal stability of the dispersedparticles. Additives used may be, for example, electrostatic or stericdispersing agents or electrolytes such as ammonium halides, ammoniumnitrates, organic ammonium salts, or salts of organic acids such asacetates, citrates, oxalates, and tartrates.

The particles are dispersed through milling in a ball mill with andwithout stirrer, stirring in a dissolver, dispersing by shearing in anultraturrax device, an ultrasonic bath, or an ultrasonic sonotrode. Thepowder agglomerates are broken up by the dispersing power and definedagglomerate sizes are obtained.

The suspensions may furthermore be mixed with additives which modify theflow properties of the suspensions and give them thixotropic properties.Additives used for this may be small quantities of organic, solublepolymers such as polyvinyl alcohol, polyacrylate derivatives,associatively acting thickeners, or fully dispersed colloidalsubstances.

Very fine-particle colloids with a particle diameter of approximately 10nm may also be used for a controlled agglomerate formation, whichcolloids in an aqueous solution exhibit a surface charge which isopposed to the surface charge of the particles used.

The suspensions obtained by these various methods may be provided on theprotective layer 5 of the front plate 1 by means of a wide variety ofprocesses. A continuous layer may be provided by spin coating, meniscuscoating, or blade coating. If a structured layer is desired, printingprocesses such as silk-screen printing or flexo printing may be used.

The layers thus provided are treated with an air flow, heat, infraredradiation, or combinations thereof for drying purposes. To prevent theformation of cracks in the layer owing to shrinkage, the drying processis carried out sufficiently slowly. The layers are given a thermalaftertreatrnent for removing additives such as the electrolytes, thedispersing agent, or the polymers. The additives can be removed withoutresidues through heating of the layers at 450° C. It may be necessary insome cases to use temperatures of 600° C. for achieving a full pyrolysisof the polymers. If the suspension provided comprises a precursor of aparticle according to the invention, the relevant conversion will alsotake place during the thermal treatment.

Subsequently, the front plate 1 together with further components suchas, for example, a back plate 2 with address electrodes 11 covered withphosphor layers 10 in one of the three basic colors red, green, or blue,and a mixture of rare gases, is used for the manufacture of an AC plasmapicture screen.

Preferably, the UV light reflecting layer 8 is used in AC plasma picturescreens in which the plasma cells are excited by an AC voltage and thedischarge electrodes 6, 7 are covered with a dielectric layer 4. Inprinciple, however, a UV light reflecting layer may also be used for DCplasma picture screens in which the discharge electrodes 6, 7 are notcovered by a dielectric layer 4.

FIG. 2 shows the reflection characteristic of a UV light reflectinglayer according to the invention made of SiO₂ with particle diameterslying between 10 and 110 nm and a layer thickness of 3 μm. The maximumfor the layer reflectivity lies in the range of 170 nm. This isparticularly advantageous because a major portion of the UV radiation ina xenon discharge is emitted at 172 nm. The reflectivity of the UV lightreflecting layer is clearly lower in the wavelength range correspondingto visible light.

Embodiments of the invention will be explained in more detail below,representing examples of how the invention may be implemented inpractice.

Embodiment 1

The pH value of a solution of 4 g Mg(NO₃)₂·6H₂O in 100 ml H₂O was setfor 11.0 through the introduction of NH₃ for a period of 4 h. ColloidalMg(OH)₂ particles were formed thereby with an average particle diameterof 350 nm. The suspension was mixed with 10 ml of a 10% pigmentdispersing agent solution and 10 ml of a 10% associative thickenersolution under stirring. A layer of Mg(OH)₂ particles was provided onthe protective layer 5 of a front plate 1 by means of spin coating,which front plate comprises a glass plate 3, a dielectric layer 4 ofglass containing PbO, a protective layer 5 of MgO, and two dischargeelectrodes 6, 7 made of ITO. Then the entire front plate 1 was dried,given an aftertreatment at 450° C. for 2 h, and the Mg(OH)₂ wasconverted into MgO. The layer thickness of the UV light reflecting layer8 of MgO was 1.5 μm. The front plate 1 was subsequently used forassembling a plasma picture screen which showed an enhanced luminance.

Embodiment 2

100 ml of a 10% colloidal suspension of SiO₂ particles with a particlediameter of 200 nm were mixed with 10 ml of a 10% pigment dispersingagent solution and 20 ml of a 10% associative thickener solution. Theentire mixture was thoroughly mixed. A layer of SiO₂ particles wasprovided as a UV light reflecting layer 8 on the protective layer 5 of afront plate 1 by means of spin coating, which front plate comprises aglass plate 3, a dielectric layer 4, a protective layer 5, and twodischarge electrodes 6, 7. The dielectric layer 4 comprised glasscontaining PbO, the protective layer 5 comprised MgO, and the twodischarge electrodes 6, 7 were made of ITO. The entire front plate 1 wasdried and given an aftertreatment at 450° C. for two hours. The layerthickness of the UV light reflecting layer 8 of SiO₂ was 0.5 μm. Thefront plate 1 was subsequently used for assembling a plasma picturescreen which showed an enhanced luminance.

Embodiment 3

10 g aluminum isopropylate were added to 150 ml H₂O which had previouslybeen heated to 70° C. The mixture was heated for 2 h in a reflow processand subsequently mixed with 0.5 ml glacial acetic acid. The entiremixture was then heated once more for 10 hours in a reflow process. Asuspension was obtained which contained Al₂O₃ particles with an averageparticle diameter of 300 nm. After the addition of 10 ml of a 10%associative thickener solution, the resulting suspension was thoroughlymixed. A layer of Al₂O₃ particles was provided as a UV light reflectinglayer 8 on the protective layer 5 of a front plate 1 by means of spincoating, which front plate comprises a glass plate 3, a dielectric layer4 of glass containing PbO, a protective layer 5 of MgO, and twodischarge electrodes 6, 7 made of ITO. The entire front plate 1 wasdried and given an aftertreatmnent at 450° C. for two hours. The layerthickness of the UV light reflecting layer 8 of Al₂O₃ was 0.8 μm. Thefront plate 1 was subsequently used for assembling a plasma picturescreen which showed an enhanced luminance.

Embodiment 4

A solution of 4 g Mg(NO₃)₂·H₂O and 1.2 g NH₄F in 100 ml H₂O was set fora pH value of 7.5 through the addition of 2M soda lye. The pH value wasraised to 11.0 through the introduction of NH₃ during a period of 4 h.MgF₂ particles with a particle diameter of on average 400 nm were formedthereby. The resulting suspension was mixed with 10 ml of a 10% pigmentdispersing agent solution and 10 nm of a 10% associative thickenersolution. A layer of MgF₂ particles was provided as a UV lightreflecting layer 8 on the protective layer 5 of a front plate 1 by meansof spin coating, which front plate comprises a glass plate 3, adielectric layer 4, a protective layer 5, and two discharge electrodes6, 7. The dielectric layer 4 comprised glass containing PbO, theprotective layer 5 comprised MgO, and the two discharge electrodes 6, 7were made of ITO. The entire front plate 1 was dried and given anaftertreatment at 450° C. for two hours. The layer thickness of the UVlight reflecting layer 8 of MgF₂ was 1.0 μm. The front plate 1 wassubsequently used for assembling a plasma picture screen which showed anenhanced luminance.

Embodiment 5

The pH value of a solution of 2.0 g Ca(NO₃)₂·4H₂) in 50 ml H₂O was setfor 7.5 through the addition of 2M soda lye. This solution was slowlydripped into a solution of 1.7 g sodium metaphosphate (Graham's salt) in50 ml H₂O. A suspension in which calcium phosphate particles with aparticle diameter from 270 nm to 290 nm were present was obtained afterone hour of stirring. The suspension was mixed with 10 ml of a 10%pigment dispersing agent solution under stirring. A layer of Ca₃(PO₄)₂particles was provided on the protective layer 5 of a front plate 1 as aUV light reflecting layer 8 by means of spin coating, which front platecomprises a glass plate 3, a dielectric layer 4, a protective layer 5,and two discharge electrodes 6, 7. The dielectric layer 4 comprisedglass containing PbO, the protective layer 5 comprised MgO, and the twodischarge electrodes 6, 7 were made of ITO. The entire front plate 1 wasdried and given an aftertreatment at 450° C. for two hours. The layerthickness of the UV light reflecting layer 8 of Ca₃(PO₄)₂ was 0.7 μm.The front plate 1 was subsequently used for assembling a plasma picturescreen which showed an enhanced luminance.

Embodiment 6

150 g Al₂O₃ prepared by flame pyrolysis and having a particle diameterof up to 200 nm was slowly stirred into a 0.005 molar solution ofammonium acetate in 500 ml distilled water with a stirrer. Thesuspension resulting from the above was treated for 15 minutes in anultrasonic sonotrode. The suspension was mixed with 25.0 ml of a 4.7%solution of polyvinyl alcohol in water under stirring. The suspensionwas subsequently purified by filtration.

A layer of Al₂O₃ particles was provided as a UV light reflecting layer 8on the protective layer 5 of a front plate 1 by means of spin coating,which front plate comprises a glass plate 3, a dielectric layer 4, aprotective layer 5, and two discharge electrodes 6, 7. The dielectriclayer 4 comprised PbO, the protective layer 5 comprised MgO, and the twodischarge electrodes 6, 7 were made of ITO. The front plate 1 was firstdried and then subjected to a thermal aftertreatment at 450° C. for twohours. The layer thickness of the UV reflecting layer 8 of Al₂O₃ was 2.0μm. The front plate 1 was subsequently used for assembling a plasmapicture screen which showed an enhanced luminance.

Embodiment 7

A solution of 21.4 mg ammonium chloride p.a. in 400 g distilled waterwas stirred at room temperature with a stirrer and slowly mixed with 200g pyrogenic silicic acid with a particle diameter of between 10 and 110nm. After this addition had been completed, the resulting suspension wasplaced in an ultrasonic bath for 60 minutes, during which the suspensionwas continuously stirred. Under stirring, the suspension was mixed with5.0 ml of a 1% aqueous polymer solution of an associative thickenerwhich bad been set for a pH value of 9.5 by means of ammonia gas. Thesuspension thus obtained was subsequently purified by filtration.

A layer of SiO₂ particles was provided as a UV light reflecting layer 8on the protective layer 5 of a front plate 1 by means of spin coating,which front plate comprises a glass plate 3, a dielectric layer 4, aprotective layer 5, and two discharge electrodes 6, 7. The dielectriclayer 4 comprised glass containing PbO, the protective layer 5 comprisedMgO, and the two discharge electrodes 6, 7 were made of ITO. The frontplate 1 was first dried and subsequently subjected to a thermalaftertreatment at 450° C. for two hours. The layer thickness of the UVreflecting layer 8 of SiO₂ was 3.0 μm. The front plate 1 wassubsequently used for assembling a plasma picture screen which showed anenhanced luminance.

Furthermore, a cleaned quartz plate was coated with the suspension in aspin coating process. After drying in a drying cabinet and anaftertreatment of the coated quartz plate at 450° C. in air, atransparent SiO₂ layer with a layer thickness of 3.0 μm was obtained.The reflection of this SiO₂ layer as a function of the wavelength over arange from 100 nm to 600 nm is plotted in FIG. 2.

Embodiment 8

A 0.003 molar solution of 250 g analytical grade (p.a.) armoniumfluoride in distilled water was stirred at room temperature with astirrer and slowly mixed with 100 g amorphous SiO₂ with a particlediameter of 20 nm. After this addition had been completed, the resultingsuspension was treated with an ultraturrax stirring rod for 10 minutes.While stirring, the suspension was mixed with 50 ml of a 10% aqueoussuspension of a fully dispersed, pyrogenic silicic acid with a particlesize between 10 and 20 nm. An aqueous solution of 33 g polyacryl amidewas also added to the suspension under stirring. The resultingsuspension was then cleaned by filtration.

A layer of SiO₂ particles was provided as an UV light reflecting layer 8on the protective layer 5 of a front plate 1 by means of spin coating,which front plate comprises a glass plate 3, a dielectric layer 4, aprotective layer 5, and two discharge electrodes 6, 7. The dielectriclayer 4 comprised glass containing PbO, the protective layer 5 comprisedMgO, and the two discharge electrodes 6, 7 were made of ITO. The frontplate 1 was first dried and then subjected to a thermal aftertreatmentat 490° C. for 2 h. The layer thickness, of the UV reflecting layer 8 ofSiO₂ was 2.5 μm. The front plate 1 was subsequently used for assemblinga plasma picture screen which showed an enhanced luminance.

What is claimed is:
 1. A plasma picture screen, comprising: a frontplate; a back plate; a plurality of gab-filled plasma cells arrangedbetween the front and back plates and separated by partitioning walls;and a plurality of electrodes on the front plate and the back plate forgenerating corona discharges, wherein the front plate includes a glassplate on which a dielectric layer, a protective layer and a UV lightreflecting layer are provided, the protective layer is between thedielectric layer and the UV light reflecting layer, wherein the UV lightreflective layer covers only a portion of the protective layer.
 2. Aplasma picture screen as claimed in claim 1, wherein the UV lightreflecting layer includes oxides of the composition M₂O, such as Li₂O,or oxides of the composition MO, with M chosen from the group Mg, Ca,Sr, and Pa, or oxides of the composition M₂O₃, with M chosen from thegroup B, Al, Sc, Y, and La, or oxides of the composition MO₂, with Mchosen from the group Si, Ge, Sn, Ti, Zr, and Hf, or oxides of thecomposition M′M″₂O₄, with M′ chosen from the group Mg, Ca, Sr, and Pa,and M″ chosen from the group Al, Sc, Y, and La, or fluorides of thecomposition MF, with M chosen from the group Li, Ka, K, Rb, Cs, and Ag,or fluorides of the composition MF₂, with M chosen from the group Mg,Ca, Sr, Ba, Sn, Cu, Zn, and Pb, or fluorides of the composition MF₃,with M chosen from the group La, Pr, Sm, Eu, Gd, Yb, and Lu, orfluorides of the composition M′M″F₃, with M′ chosen from the group Li,Na, K, Rb, and Cs, and M″ chosen from the group Mg, Ca, Sr, and Ba, orphosphates of the composition M₃PO₄, with M chosen from the group Li,Na, K, Rb, and CB, or phosphates of the composition M₃(PO₄)₂, with Mchosen from the group Mg, Ca, Sr, and Ba, or phosphates of thecomposition MPO₄, with M chosen from the group Al, Sc, Y, La, Pr, Sm,Eu, Gd, Yb, and Lu, or phosphates of the composition M₃(PO₄)₄, with Mchosen from the group Ti, Zr, and Hf, or metaphosphates with a chainlength n of between 3 and 9 and the composition (M_(x)PO₃)_(n), with x=1if M is chosen from the group Li, Na, K, Rb, and Cs, x=½ if M is chosenfrom the group Mg, Ca, Sr, Ba, Sn, Cu, Zn, and Pb, x=⅓ if M is chosenfrom the group Al, Sc, Y, La, Pr, Sm, Eu, Gd, Yb, and Lu, and x=¼ if Mis chosen from the group Ti, Hf, and Zr, or polyphosphates with a chainlength n between 10¹ and 10⁶ and the composition (M_(x)PO₃)_(n), withx=1 if M is chosen from the group Li, Na, K, Rb, and Cs, x=½ if M ischosen from the group Mg, Ca, Sr, Ba, Sn, Cu, Zn, and Pb, x=⅓ if M ischosen from the group Al, Sc, Y, La, Pr, Sm, Eu, Gd, Yb, and Lu, and x=¼if M is chosen from the group Ti, Hf, and Zr, or primary phosphates ofthe composition MH₂PO₄, with M chosen from the group Li, Na, K, Rb, andCs, or NH₄H₂PO₄, or diamond.
 3. A plasma picture screen as claimed inclaim 1, wherein the UV light reflecting layer includes particles with aparticle diameter of between 200 nm and 500 nm.
 4. A plasma picturescreen as claimed in claim 3, wherein the UV light reflecting layer hasa thickness of 0.5 μm to 5 μm.
 5. A plasma picture screen as claimed inclaim 1, wherein the UV light reflecting layer comprises agglomerates ofparticles having particle diameters of between 10 nm and 200 nm.
 6. Aplasma picture screen as claimed in claim 5, wherein the UV lightreflecting layer has a thickness of 0.2 μm to 10 μm.